8.7 - 8.10 Flashcards
Note 1 —-»
For a plant or an animal to grow, develop normally, and maintain its tissues once fully grown, the timing of cell division in different parts of its body must be carefully controlled. For example, your skin cells and the cells lining your digestive tract divide frequently, replacing cells that are constantly being abraded and sloughed off. In contrast, cells in your liver usually do not divide unless the liver is damaged. Other cells, such as your muscle cells, never divide. By growing animal cells in culture—that is, in a laboratory growth medium, outside of the body—researchers have been able to identify many factors, both chemical and physical, that influence cell division. For example, most animal cells exhibit anchorage dependence; they must be in contact with a solid surface—such as the inside of a culture dish or the extracellular matrix of a tissue—to divide. Another physical factor that can regulate growth rate is density-dependent inhibition, a phenomenon in which crowded cells stop dividing. Animal cells growing on the surface of a dish multiply to form a single layer and usually stop dividing when they touch one another. If some cells are removed, those bordering the open space begin dividing again and continue until the vacancy is filled. What actually causes the inhibition of growth? Studies of cultured cells suggest that physical contact of cell-surface proteins between adjacent cells is responsible for inhibiting cell division.
Anchorage Dependence
The requirement that to divide, a cell must be attached to a solid surface.
Density-dependent inhibition
The ceasing of cell division when cells touch one another.
Note 2 —-»
Chemical factors can also influence the rate of cell growth. For example, when grown in the laboratory, cells fail to divide if an essential nutrient is left out of the culture medium. Additionally, most types of mammalian cells will divide only if certain proteins, called growth factors, are present.
Note 3 —-»
A protein called vascular endothelial growth factor (VEGF) stimulates the growth of new blood vessels during fetal development and after injury.
Growth Factors
A protein secreted by certain body cells that stimulate other cells to divide.
Note 4 —-»
The importance of proper cell division to your own health becomes clear when you consider what happens if control is lost. Cancerous cells are different from normal body cells because they no longer exhibit the types of regulation discussed here. For example, cancer cells are not subject to anchorage dependence; they grow whether or not they are in contact with a suitable surface. Additionally, density-dependent inhibition fails in tumors; cancer cells continue to divide even at high densities, piling up on one another. The importance of proper cell division to your own health becomes clear when you consider what happens if control is lost. Cancerous cells are different from normal body cells because they no longer exhibit the types of regulation discussed here. For example, cancer cells are not subject to anchorage dependence; they grow whether or not they are in contact with a suitable surface. Additionally, density-dependent inhibition fails in tumors; cancer cells continue to divide even at high densities, piling up on one another.
Compared with a control culture, the cells in an experimental culture are fewer but much larger in size when they cover the dish surface and stop growing. What is a reasonable hypothesis for this difference?
The experimental culture is deficient in one or more growth factors.
Note 5 —-»
The reproductive behavior of cells—whether to divide or not—results from interactions among many different molecules. In a living animal, most cells are anchored in a fixed position and bathed in a solution of nutrients supplied by the blood, yet they usually do not divide unless they are signaled by other cells to do so. Growth factors are the main signals, and their role in promoting cell division leads us back to our earlier discussion of the cell cycle.
Note 6 —-»
The sequential events of the cell cycle are directed by a distinct cell cycle control system, represented by the gray circle in the center of the art. The thin gray bar extending from the circle represents the current position in the cell cycle. The cell cycle control system is a set of molecules that both triggers and coordinates key events in the cell cycle. The cell cycle is not like a row of falling dominoes, with each event causing the next one in line. During mitosis, for example, metaphase does not automatically lead to anaphase. Instead, proteins of the cell cycle control system must trigger anaphase to begin.
Cell Cycle Control System
A cyclically operating set of proteins that triggers and coordinates events in the eukaryotic cell cycle.
At which of the three checkpoints described in this module do the chromosomes exist as duplicated sister chromatids?
G2 and M checkpoints
Note 7 —-»
Cancer, which claims the lives of one out of every five people in the United States, is a disease of the cell cycle. Cancer cells do not heed the normal signals that regulate the cell cycle; they divide excessively and invade other tissues of the body. If unchecked, cancer cells may continue to grow and spread until they kill the organism. Cancer begins when a single cell undergoes changes that convert a normal cell to a cancer cell. Such a cell often has altered proteins on its surface and the body’s immune system normally recognizes the cell as an alien and destroys it. However, if the cell evades destruction, it may multiply to form a tumor, a mass of abnormally growing cells within otherwise normal tissue. If the abnormal cells remain at their original site, the lump is called a benign tumor. Benign tumors can cause problems if they grow in and disrupt certain organs, such as the brain, but often they can be completely removed by surgery or even (in cases in which they pose no imminent threat) left alone.
Note 8 —-»
In contrast, a malignant tumor is a mass of abnormally reproducing cells that can spread into neighboring tissues and invade other parts of the body, with the potential to displace normal tissue and interrupt organ function as it grows. An individual with a malignant tumor is said to have cancer. Cancer cells may separate from the original tumor or secrete signal molecules that cause blood vessels to grow toward the tumor. A few tumor cells may then enter the blood and lymph vessels and thereby move to other parts of the body, where they may proliferate and form new tumors. The spread of cancer cells beyond their original site is called metastasis.
Tumor
Abnormal mass of rapidly growing cells that forms within otherwise normal tissue.